There is a simple proof that it is impossible to produce electricity using Earth’s rotation through the nonrotating component of its own magnetic field. However, the authors have identified a loophole in that proof, and it appears that power generation could be possible in a laboratory system. Experimental verification of this result could carry implications for a clean-energy future.

The nuclear spin-lattice relaxation time $T_1$ is an important probe of the electronic properties of solids, but here traditional NMR methods struggle due to weak signals, so advanced tools like magnetic resonance force microscopy (MRFM) are needed. The authors extend high-resolution MRFM to measure $T_1$ at a temperature of 42 mK, a 100-fold improvement, with a 1000-fold increase in volume sensitivity. This opens up the possibility to measure the magnetic properties of oxide interfaces, topological insulators, high-$T_c$ superconductors, and other strongly correlated electron systems.

Conventional wisdom suggests that when a beam of light illuminates a particle, the radiation pressure pushes it in the direction of the light flow, $i.e.$ downstream. However, under the right conditions, the particles can be made to move $upstream$, toward the light source. Here the authors describe a means to transport engineered chiral nanoparticles with an “optical conveyor belt” that can move in $either$ $direction$. Simply controlling the helicity of the incoming wave enables switching between persistent attractive or repulsive optical forces, with just one beam and no optical traps.

Realizing a scalable quantum network based on defects in diamond is impeded by the difficulty of collecting the photons that are useful for generating spin-spin entanglement. By integrating the diamond $NV$ center with a single-mode waveguide in a $III-V$ semiconductor photonic circuit, the authors achieve photon collection rates above the theoretical limit for free-space, nonresonant collection. This performance meets or exceeds that of all-diamond photonic circuits, and along with the scale of integration shows the potential of this hybrid platform for quantum information processing.

The familiar Shockley-Queisser limit of about 34% efficiency only applies to a single-junction solar cell; in principle, a cell with an infinite number of junctions could attain 87%. Luminescent coupling (LC) between subcells in a multijunction photovoltaic device is a key factor in its optimization. The authors explain how pump-probe spectral photovoltage measurements can help quantify LC easily. This simple method could be readily incorporated into active photovoltaic research laboratories worldwide.

Applications in optics and photonics, such as frequency doubling of a laser, require compact, efficient hardware. The authors demonstrate highly efficient nonlinear frequency conversion in a microdisk resonator about 100 $\mu$m across. The key to efficiency in this parametric process is phase matching, which becomes more difficult at smaller length scales. This achievement is a significant step toward chip-integrated nonlinear optical devices.

Topologically protected states are desired for noise-resistant logic applications, but are our expectations realistic? In samples featuring quantum Hall edge modes, nonlocal transport is quite resilient to disorder and inelastic scattering, compared to local transport in the same samples. This study shows, however, that nonlocal transport via quantum $spin$ Hall edge modes is not all it’s cracked up to be. Unfortunately, these modes present deficiencies that seem to render them impractical for low-power information processing.

Efficient single-photon emitters (SPEs) are key to quantum communications and information processing, and frequency tunability is an especially appealing feature. The authors find that at 100 K—above liquid-nitrogen temperature—piezoelectric (PZ) InAs quantum dots exhibit much greater Stark-effect tunability of their excitonic emissions than do non-PZ dots. The observed redshifts cannot be explained unless $nonlinear$ PZ effects are taken into account. Beyond SPEs, these results are also important for understanding the behavior of strained nanostructures grown along polar crystal directions.

In recent years the LaAlO${}_3$/SrTiO${}_3$ (LAO/STO) interface has been a playground of condensed matter physics, as it exhibits exotic electronic properties not seen in either constituent. Furthermore, combining this system with graphene has its own charm: The photoconductivity of graphene is a good probe of the natural polar field within the LAO layer, as evidenced by hole doping in a graphene/LAO/STO hybrid system under pulsed deep-ultraviolet illumination. This also renders graphene/LAO/STO a convenient deep-ultraviolet sensor, and suggests its use in broad-spectrum photodetectors.

There is growing interest in electronic circuit elements that “remember” their prior states: memristors, memcapacitors, and meminductors. These generalize the familiar resistors, capacitors, and inductors, and are important in the development of ultradense, nonvolatile data storage, as well as neuromorphic computing. The authors propose $quantum$ realizations of memory circuit elements, based on solid-state qubits. The quantum properties of qubit-based systems introduce a new aspect of functionality to the toolbox of memory devices.

The nuclear spin-lattice relaxation time $T_1$ is an important probe of the electronic properties of solids, but here traditional NMR methods struggle due to weak signals, so advanced tools like magnetic resonance force microscopy (MRFM) are needed. The authors extend high-resolution MRFM to measure $T_1$ at a temperature of 42 mK, a 100-fold improvement, with a 1000-fold increase in volume sensitivity. This opens up the possibility to measure the magnetic properties of oxide interfaces, topological insulators, high-$T_c$ superconductors, and other strongly correlated electron systems.

Topological insulators support helical edge states, exotic excitations with one-way propagation and protection against scattering. In terms of applications, these states are usually considered in the context of quantum information processing. The author, however, proposes a rather different, $classical$ realization of a topological insulator, engineered using piezoelectric resonators. The room-temperature edge states of such a system point to compact, nonreciprocal microwave components for $e.g.$ mobile phones.

Lately binary tin sulfides, being composed of inexpensive earth-abundant elements, have come under intense scrutiny for applications spanning photovoltaics, thermoelectrics, valleytronics, batteries, and photocatalysis. The authors use first-principles methods to examine these materials’ point defects, which can play crucial roles in electronic applications. This comprehensive study expands the technological horizons for the tin sulfides, particularly the less-studied Sn${}_2$S${}_3$, which is predicted to support either $p$-type or $n$-type doping—a coveted property.

Silicon carbide is used in a wide variety of applications, including high-voltage transistors. High-temperature oxidation is commonly used to improve charge-carrier lifetime in hexagonal SiC, by annihilating carbon-vacancy defects. However, a combination of detailed measurements and advanced modeling shows that the oxidation itself introduces defects that act as weak recombination centers. This physical understanding helps to direct the materials engineering needed to advance SiC electronics.

Optoelectronic hardware based on excitons (weakly bound electron-hole pairs) would combine the processing speed of photons with the integration density of modern electronics. Practical devices need to work at room temperature, though, where thermal energy can wreck excitons before they travel far enough to transmit information usefully. The authors demonstrate propagation of indirect excitons in polar (Al,Ga)N/GaN quantum wells over distances of several micrometers at 300 K. These results point the way to working devices based on gate-controlled exciton transport in wide-band-gap semiconductors.

Nanoelectromechanical oscillators are conventionally operated near their resonant frequency, where they are ultrasensitive to tiny signals. The authors show that, counterintuitively, a SiC nanowire resonator coupled to an overdamped electrical circuit, and driven above both resonance and cutoff frequencies, gives $better$ performance than one with conventional tuning, and can self-oscillate. This method is simple and can be generalized to any electromechanical system, for $e.g.$ force sensing, or studying synchronization phenomena.

The coupling of electromagnetic fields to nanomechanical systems has ushered in the field of $quantum$ $optomechanics$, in which vibrating objects can be studied and used at the level of their quantum zero-point motion. The authors fabricate a planar technology platform for integrating nanophotonic, nanomechanical, and superconducting microwave circuits. Joining these components could yield a quantum converter between the microwave and optical frequency domains, enabling long-range networks of superconducting qubits for quantum information processing.

Learning the state of a multilevel quantum system without changing that state, as in quantum computing, is tricky. Ordinarily a dispersive measurement of such a system destroys all coherence. By exploiting the two excited states of a three-level system (qutrit), the authors show how to determine whether it is in the ground state or an excited state, while preserving coherence between the excited states. This $degenerate$ $measurement$ can be used to detect leakage errors, and to test quantum contextuality, the critical resource behind the exponential speedup of a quantum computer.

Being a good thermoelectric material is a balancing act between high electrical conductivity $\sigma$ and low thermal conductivity $\kappa$, because both quantities depend directly on the flow of electrons. Fortunately, $\kappa$ $also$ depends on lattice phonons, so this contribution can be cut—for example, by scattering from randomly distributed heavy atoms. The authors’ calculations show that adding tin to alloys of silicon and germanium should yield quite good thermoelectrics, especially in thin-film systems.

Conventional wisdom suggests that when a beam of light illuminates a particle, the radiation pressure pushes it in the direction of the light flow, $i.e.$ downstream. However, under the right conditions, the particles can be made to move $upstream$, toward the light source. Here the authors describe a means to transport engineered chiral nanoparticles with an “optical conveyor belt” that can move in $either$ $direction$. Simply controlling the helicity of the incoming wave enables switching between persistent attractive or repulsive optical forces, with just one beam and no optical traps.

There is a simple proof that it is impossible to produce electricity using Earth’s rotation through the nonrotating component of its own magnetic field. However, the authors have identified a loophole in that proof, and it appears that power generation could be possible in a laboratory system. Experimental verification of this result could carry implications for a clean-energy future.

Shining a short, intense laser pulse on micrometer-sized droplets of liquid metal creates a plasma that is a bright source of extreme ultraviolet (EUV) light. The authors study in detail the propulsion and deformation of such droplets due to a laser “kick”, and unveil the underlying mechanisms and scaling laws. Optimizing EUV plasma sources for next-generation nanolithography requires a deep understanding of both droplet-laser coupling and droplet fluid-dynamical response, which this work provides.

Engineering the photonic interface with spins in color centers, such as the $NV$ center in diamond or the neutral divacancy in silicon carbide, is crucial for applications in quantum communication and precision sensing. Exploiting SiC’s compatibility with conventional thin-film fabrication, the authors use on-chip nanoscale photonic crystal cavities to dramatically enhance the measurement of the spin and optical properties of color-center ensembles. These results bring us a step closer to realizing scaled-up defect-based quantum technologies.